Consider turning bindgen into its own separate project

[atomicptr]

The bindgen command probably shouldn't be part of the main build.zig but instead have its own sub project and have zig build generate just be a standalone build step that builds and calls bindgen.

Rough idea:

bindgen/
    src/       - bindgen source files
    build.zig

this has the added advantage of being able to remove zig-json as a dependency from godot-zig.